Modern automotive electrical systems typically include an alternator which is rotated by the vehicle's engine to generate an electrical voltage for charging the vehicle's battery and for operating the numerous electrical loads which are powered by battery voltage.
A voltage regulation system is used to control the output power delivered by the alternator system. Since the voltage generated by the rotating alternator armature can be controlled to some extent by varying the amount of current delivered to the alternator's field windings, a first voltage regulating mechanism is typically used to reduce the alternator field current whenever the generated output voltage exceeds a predetermined level. As long as the alternator output voltage is below that predetermined level, this first regulating mechanism maintains the field current at a predetermined level equivalent to the maximum field current which the alternator can safely handle.
The alternator preferably delivers an output voltage substantially above the battery's terminal voltage. Devices which can be more efficiently powered at higher voltages, such as blower motors and windshield heating elements, may be powered directly from the higher voltage produced by the alternator, while the low voltage devices are powered by a more precisely regulated voltage produced by the switching converter having an output voltage which is regulated to a level appropriate for charging the vehicle's battery.
The switching converter adjusts its output voltage to the desired level by varying the switching times or "duty cycle" of the transistors which control the flow of current to the load. A regulating feedback circuit responds to any deviation in the output voltage from a preset reference level by producing an error signal which alters the duty cycle to maintain the desired output voltage. U.S. Pat. No. 4,694,238 issued on Sep. 7, 1987 to Peter Norton discloses such an electrical supply system in which a first regulator controls the alternator field current and a second varies the switching duty cycle of a solid-state switching converter to maintain desired power output voltages.
If the demands of the connected load circuit exceed the output capabilities of the regulated alternator system, the desired output voltage cannot be maintained. Under certain conditions, increasing the converter's duty cycle in a futile attempt to increase its output voltage actually reduces the alternator's ability to deliver power to the vehicle's electrical system.